[3/4,next] net: lan78xx: Switch to SPDX identifier

i don't think we can do this. I'm simply following this guideline [1].

How about this:

// SPDX-License-Identifier: GPL-2.0+
/*
 * Copyright (C) 2015 Microchip Technology
 */

[1] - https://www.kernel.org/doc/html/v4.18/process/license-rules.html

Thank you.
Looks good to me.

Thanks,
-Raghu